1. Field of the Invention
The present invention relates to an elastic wave device and a method for manufacturing the same. The present invention particularly relates to a multilayer structure of an IDT (inter-digital transducer) electrode of the elastic wave device.
2. Description of the Related Art
Elastic wave devices are actually used as, for example, band-pass filters mounted in RF circuits for communication devices such as mobile phones. The elastic wave devices are required to have predetermined frequency characteristics so as to function as band-pass filters and are also required to have electric power handling capability so as to resist electricity applied to the RF circuits.
An elastic wave device includes a piezoelectric substrate and IDT electrodes located on the piezoelectric substrate. Each IDT electrode includes electrode fingers and a bus bar. The IDT electrode is formed by stacking, for example, a first electrode film which has low electrical resistance and which is made of Al (aluminium) and a second electrode film with relatively high density. When large electricity is applied to the elastic wave device, the IDT electrode generates heat and therefore interdiffusion occurs between the first and second electrode films. The interdiffusion therebetween increases the electrical resistance of the IDT electrode to deteriorate frequency characteristics of the elastic wave device.
International Publication No. WO 2009/016906 discloses an elastic wave device including an electrode finger 112A as shown in FIG. 7. The electrode finger 112A includes an adhesion film 121, a second electrode film (Pt film) 122, a Ti film 125, a first electrode film (Al alloy film) 126, and a Ti film 128 stacked on a piezoelectric substrate 111 in that order. In the elastic wave device, since the Ti film 125 is located between the first electrode film 126 and the second electrode film 122, the interdiffusion between the first electrode film 126 and the second electrode film 122 can be suppressed to a certain extent. However, in the case where the elastic wave device is used over a long period of time, interdiffusion gradually proceeds to deteriorate frequency characteristics of the elastic wave device. That is, it is difficult to allow the elastic wave device to function over a long period of time using the Ti film 125 only.
International Publication No. WO 2012/036178 discloses an elastic wave device including an electrode finger 212A as shown in FIG. 8. The electrode finger 212A includes an adhesion film 221, a first electrode film (Al alloy film) 226, a Ti oxide film 224, and a second electrode film (Pt film) 222 stacked on a recessed portion 211C of a piezoelectric substrate 211 in that order. In the elastic wave device, since the Ti oxide film 224 is located between the first electrode film 226 and the second electrode film 222, the interdiffusion between the first electrode film 226 and the second electrode film 222 can be considerably suppressed. Therefore, frequency characteristics of the elastic wave device are unlikely to be deteriorated. However, when large electricity is applied to the elastic wave device, stress migration occurs in the first electrode film 226, which mainly contains Al, to electrically deteriorate the first electrode film 226, leading to the increase in electrical resistance of the first electrode film 226. Therefore, the insertion loss of electricity applied to the elastic wave device is increased, leading to a problem that power durability life is reduced. That is, it is difficult to allow the elastic wave device to function over a long period of time using the Ti oxide film 224 only.